FIG. 1 shows a conventional condenser microphone. A cylindrical housing 11 open at both ends has a flange 12 formed integrally therewith and extending inwardly from its front marginal edge. Diaphragm retaining rings 13 and 14 are urged and held against the flange 12 on the inside thereof. The peripheral portion of a diaphragm 15 is clamped between the diaphragm retaining rings 13 and 14. A cylindrical presser 16 is pressed forwardly against the back of the diaphragm 15. The inner surface of the housing 11 has cut therein screw threads 17, with which a ring-shaped screw 18 is threadably engaged to fix the diaphragm retaining rings 13 and 14 while pressing them forwardly. Further, ring-shaped screws 19 and 21 are threadably engaged with the screw threads 17, by which the cylindrical presser 16 is urged against the diaphragm 15, applying thereto a desired tensile force.
A back electrode 22 is disposed just behind the diaphragm 15 in opposing relation thereto and supported at the rear by a ring-shaped support plate 23 of an insulating material, which is in turn held by the ring-shaped screw 21 threadably engaged with the screw threads 17. A spacer 20 is interposed between the cylindrical presser 16 and the support plate 23, defining the space between the diaphragm 15 and the back electrode 22. A ring-shaped screw 24 is threadably engaged with the screw threads 17 behind the ring-shaped screw 21. The back electrode 22 has a terminal 25. The housing 11 is covered all over its front open end with a grid 26. The back electrode 22 is deposited with an electret film 27 opposite the diaphragm 15.
The diaphragm 15 of the conventional condenser microphone is pressed by the cylindrical presser 16 and is held taut with a predetermined tensile force. Since the condenser microphone has incorporated therein the cylindrical presser, it is inevitably bulky, calls for many assembling steps, and hence is cumbersome to assemble and expensive. Moreover, the diaphragm 15 is held taut by the cylindrical presser 16, which is retained by the ring-shaped screw 19 in the housing 11; therefore the tension of the diaphragm 15 is liable to vary with a change in ambient temperature unless the diaphragm retaining rings 13 and 14, the ring-shaped screw 18, the cylindrical presser 16 and the ring-shaped screw 19 are made of the same material. Besides, there is a risk that a change in the tension of the diaphragm 15 will occur due to a possible change in the pressure applied thereto by the cylindrical presser 16 although the latter is fixed by the two screws 19 and 21.
Furthermore, in the microphone shown in FIG. 1 the diaphragm 15 and the back electrode 22 must be spaced a predetermined distance apart with high precision. To meet this requirement, the cylindrical presser 16 and the back electrode 22 are finished to the same height (the length in the direction parallel to the axes thereof) through precision polishing, and then the space between the diaphragm 15 and the back electrode 22 is defined by the thickness of the spacer 20. In this instance, high precision is needed in machining the cylindrical presser 16 and the back electrode 22, and the spacer 20 is needed, which leads to an increase in the number of parts used. These factors inevitably raise the cost of the microphone.
An object of the present invention is to provide a simple-structured diaphragm unit which has a diaphragm held with required tension by itself and a method of making such a diaphragm unit.
Another object of the present invention is to provide a diaphragm unit designed so that the tension of the diaphragm is essentially insusceptible to the influence of temperature in the microphone housing.
Another object of the present invention is to provide a simple-structured condenser microphone having a diaphragm unit built therein.
Yet another object of the present invention is to provide a simple-structured condenser microphone which permits easy adjustment of the condenser gap.